1. Field of the Invention
The present invention relates to gas turbine engine and, more particularly, to dynamic control of the clearance between the tips of rotor blades and a surrounding shroud.
2. Description of the Prior Art
It has long been recognized that in order to maximize the overall efficiency of a gas turbine engine, the tip clearance between the rotor blades of the engine and the surrounding casing must be as small as possible. This constitutes a distinct problem in that the tip clearance between the tips of the blades and the surrounding casing varies non-uniformly with the operating conditions of the gas turbine engine. This is because the rotor blades and the casing have different thermal and centrifugal expansion characteristics. Indeed, the casing and the rotor blades are generally fabricated from material having different coefficient of expansion. Furthermore, the expansion and contraction of the casing is a function of the pressure and temperature, whereas the expansion and contraction of the rotor blades is affected by the centrifugal force and the temperatures of the blades an associated rotor disc within the various sections of the gas turbine engine.
One approach used to minimize and control the tip clearance between the rotor blades of a gas turbine engine and the surrounding casing is disclosed in U.S. Pat. No. 5,456,576 issued on Oct. 10, 1995 to Lyon. This patent teaches to surround a stage of rotor blades with a ring formed of a plurality of interconnected stiff segments supported by a hanging structure extending radially inwardly from an inner surface of the engine case.
In another attempt, U.S. Pat. No. 4,398,866 issued on Aug. 16, 1983 to Hartel et al. teaches to mount a relatively stiff split ring between a pair of opposed L-shaped rings supported within an engine case via a metallic clamping structure extending radially inwardly therefrom.
Although the tip clearance control devices described in the above-mentioned patents are effective, it has been found that there is a need for a simpler and less costly tip clearance control device which is adapted to reduce the radial space required to mount an annular shroud within an engine case about a stage of rotor blades.
It is therefore an aim of the present invention to provide a tip clearance control device which is relatively simple and economical to manufacture.
It is also an aim of the present invention to provide such a tip clearance device which contributes to minimize the overall weight of a gas turbine engine.
It is a further aim of the present invention to provide a tip clearance control device which contributes to minimize the radial dimensions of a gas turbine engine.
It is a still further aim of the present invention to provide a tip clearance control device which is adapted to efficiently isolate the engine case from the hot combustion gases flowing through a stage of rotor blades.
Therefore, in accordance with the present invention there is provided a tip clearance control device for a gas turbine engine having a shroud surrounding a stage of rotor blades. The tip clearance control device comprises a split ring adapted to be yieldingly biased radially outwardly into engagement with the shroud in order to surround the rotor blades and adjust for expansion and contraction of the shroud. The split ring is split at a single location so as to be capable of expansion and contraction during engine operation.
Also in accordance with the present invention, there is provided a tip clearance control device comprising a ring adapted to be mounted within a shroud for surrounding a stage of rotor blades. The ring has a radially inner surface defining with the tips of the rotor blades a tip clearance. The ring is split at a single location so as to be circumferentially expandable and contractible during engine operation. The ring is at least partly resilient and adapted to be biased radially outwardly in engagement with the shroud in order to prevent the ring from becoming loose within the shroud in response to radial expansion of the shroud during engine operation.
In accordance with a further general aspect of the present invention, there is provided a tip clearance control device for a gas turbine engine having a shroud surrounding a stage of rotor blades. The tip clearance control device comprises a one-piece ring adapted to be mounted within the shroud for surrounding the rotor blades at a radial distance from respective tips thereof. The one-piece ring has first and second opposed overlapping end portions formed at a single split location to provide an annular seal around the rotor blades, while allowing to adjust for thermal growth during engine operation. This arrangement advantageously reduces the cooling flow required to cool the shroud due to improved sealing as compared to conventional shroud segments.